'99 Vortec L31 won't run right

[wrager]

...despite many parts having been replaced. It will start up but then the idle will surge. After a few seconds it will idle up to about 1500, then return to a normal idle (650'ish). At that point it sounds perfectly normal, but any acceleration brings a stumble and backfire thru exhaust.

So, what I've done so far:
Replaced: Fuel pump/filter, new MPFI, plugs, new cats and O2 sensors, IAC, CTS, MAF, Evap valve, MAP, PCV

Cleaned TB, checked everywhere for vac leaks using carb spray and also propane. Timing retard is -.03. EGR delete.

Of course no codes....but the ECM has been "tuned." I thought maybe I didn't insert the new injectors correctly, but once it stops surging, it will idle fine (but no acceleration). I just don't know where to turn.